1. Field of the Invention
The present invention relates to the field of testing electronic components, and more particularly to ensuring that proper testing procedures for memory devices have been performed.
2. Description of the Related Art
Electronic circuits must operate in a variety of environments. Although typical consumer products generally operate in relatively innocuous environments at near room temperature, military equipment often is exposed to environmental extremes. For example, military radar, computers and communications equipment must be able to withstand the heat of desert combat, as well as the coldest terrestrial environments.
To ensure the reliability of its electronic equipment, the United States military requires compliance of the equipment and its components parts with so-called "Mil Specs." One such specification or standard is Mil-Std-883 group A, which governs the testing of electronic components, such as memory devices. This standard generally requires that components (also referred to as "units" or "parts") having memory storage capabilities be tested for power requirements, memory access time and other features at a number of different temperatures. The test equipment typically used to test electronic components is illustrated in FIG. 1A. The equipment includes a tester 100 having a test head 102, which communicates with the tester 100 over a cable 110. The test head 102 includes a loadboard 104. The part to be tested 106 is butted up against a contactor 105 (shown in FIG. 1B) on the loadboard 104.
FIG. 1B is a detailed illustration of a tester 100, such as the Genesis II model manufactured by Megatest Corporation. The tester 100 typically includes a keyboard 150 to allow the test operator to control the testing, and a display device 152 to display test results, among other data. The keyboard 150 and the monitor 152 are coupled to a test controller 154, which is typically implemented as a microprocessor. The test controller 154 instructs a timing generator 156 to generate various timing signals to be applied to the component 106 through pin electronics 158, loadboard 104 and contactor 105. The pin electronics 158 conforms the timing signals to the parameters of the device 106, e.g., voltage levels, slew rate, etc. Note that the pin electronics may be internal or external to the test head 102.
Under program control of the test controller 154, a parametric measurement unit (PMU) 160 applies voltage or current to the device 106 to measure the resulting current or voltage, respectively. The PMU 160 also measures the timing of the resulting signals, e.g., memory access time. The PMU 160 returns the test results to the test controller 154, which in turn may display the results on the monitor 152.
The part 106 may be pushed against the contactor 105 using a simple hand socket, as is known in the art. Based upon the test results, the test operator manually places the part in a bin container (not shown) corresponding to the test outcome.
Alternatively, the test equipment may employ a handler 180 (see FIG. 1A), such as a MCT Corporation Model 3608. The handler includes loading tubes 108, which store the parts 106 to be tested. The parts 106 are gravity fed from the loading tubes 108 to the contactor 105. Settings on the handler 180 control the temperature at which the parts 106 are tested. After a part 106 has been tested, over the interface cable 112 the tester 100 instructs handler control circuitry (not shown) in the handler 180 to direct the part 106 to an appropriate collection bin tube 114. Typically, one or more bin tubes 114 are devoted to parts that fail, while others are dedicated to collecting parts that exhibit various memory access times. After a predetermined number of parts have been tested, the collection bin tubes 114 are removed from the handler 180, and the tubes are placed in bin containers (not shown) outside of the handler. Each bin container corresponds to an associated collection bin tube category.
To comply with Mil Specs, the test equipment puts the parts through a "test flow" which may roughly be described as follows:
SORT 1 PA0 BAKE PA0 SORT 2 PA0 ASSEMBLY PA0 RAW CLASS PA0 BURN-IN PA0 PBIC (25.degree. C.) PA0 FPO-1 (128.degree. C.) PA0 QABO (125.degree. C.) PA0 FPO-2 (-58.degree. C.) PA0 QABO (-55.degree. C.) PA0 MARK PA0 PACK PA0 FQA (25.degree. C.) PA0 CONTINUITY PA0 LEAKAGE PA0 POWER PA0 PROGRAM PA0 TIMING PA0 ERASE PA0 SPECIAL FUNCTIONS PA0 BINOUT
Each step of the test flow is summarized as follows:
SORT 1
During SORT 1, all of the memory devices remain attached together in the same wafer. All memory cells are programmed to hold a charge.
BAKE
During BAKE, the wafer is heated for 72 hours at a temperature of approximately 250.degree. C.
SORT 2
During SORT 2, all memory cells are tested to make sure that none has lost the programmed charge. If any cells have lost their charge, then the wafer is discarded.
ASSEMBLY
The wafer is diced and connected to the device pins, inside a suitable electronic package.
RAW CLASS
During RAW CLASS, the devices are initially checked for broken bonds by running continuity and leakage checks on all chips.
BURN-IN
Typically, if a part is defective, it will fail during the first year of its life. Thus, during BURN-IN, the inputs of the device are toggled at a high voltage and a high temperature to simulate the effects of approximately one year of usage. If the device survives BURN-IN, then there is a high probability that the device will not fail in the future.
PBIC (25.degree. C.)
During the POST BURN-IN CHECK ("PBIC") of the test flow, all units are run through a "test sequence" at 25.degree. C. (approximately room temperature) by the tester 100. The test sequence, which is repeated in later steps at different temperatures, may roughly be described as follows:
Each step of the test sequence may be summarized as follows:
1. Continuity
The Continuity check ensures that all pins of the part 106 contact the contactor of the test head 180. The tester 100 forces a voltage onto selected pins of the part 106, and measures the resulting current. If the measured current falls within a given range, then the part 106 passes the continuity test. If, however, the current falls outside the acceptable range, then the part 106 fails the continuity check, and the tester 100 instructs the handler 180 to place the part 106 in a collection bin tube 114 for failed parts (the "failure collection bin tube").
2. Leakage
During the Leakage test step, the parts that passed the continuity check are tested to determine whether any leads are shorted together. Failed parts are directed to the failure collection bin tube, while passing parts remain in contact with the test head 180 for the next test step.
3. Power
The tester 100 causes the supply voltage specified in the parts manual to be applied to the part 106. The resulting current is measured. If the current falls outside of a predetermined range, the part is rejected and directed to the failure collection bin. Otherwise, the part is left in place against the contactor for the next test step.
4. Program
During the Program step, the tester 100 programs the nonvolatile memory device 106 with a predetermined bit pattern, preferably arranged to induce the worst case timing situation.
5. Timing
The tester 100 performs timing tests on each memory cell of the device 106 to determine the worst case memory access time. If the memory access time is unacceptable, then the part is binned out to the failure collection bin tube. Typically, within the acceptable timing range, there may be more than one acceptable memory access time depending upon the needs of the customer. For example, some customers require a 100 nanosecond memory access time, while others are satisfied with a 120 nanosecond memory access time. Accordingly, if the part 106 passes the tests following the timing test, it may be directed to one of a number of bins according to the worst case access time of the part being tested.
6. Erase
During this test step, the part 106 is erased of the programming it received during the Program step.
7. Special Functions
During this step, the tester 100 tests special chip functions. For example, the part may be checked for junction spiking, leaky columns, and other physical device defects, and binned out accordingly as having passed or failed these tests.
8. Binout
During Binout, if the part 106 has not already been binned out because of failure, it is in this step directed to the appropriate collection bin tube 114 depending upon whether the part, for example, exhibited a high, low or medium memory access speed, among other parameters.
FPO-1 (128.degree. C.)
After a part 106 has passed the PBIC (25.degree. C.) test of the test flow, it is put through the first flow process order (FPO-1) test, in which the part is tested according to the above-described test sequence at 128.degree. C. The military test specification requires that the part be operable at 125.degree. C. Therefore, the part is actually tested with a 3.degree. guard band at 128.degree. C.
QABO (125.degree. C.)
During Quality Assurance Buy Off ("QABO") testing only a sample of the entire lot of units 106 is tested at 125.degree. C. Theoretically, any parts that reach this step without failure should pass the QABO test at the non-guardbanded 125.degree. C. temperature. If a part 106 fails this test, then the testing procedure itself is suspect, and the entire lot must be retested.
FPO-2 (-58.degree. C.)
All those parts 106 that pass the QABO test are tested according to the test sequence at -58.degree. C. The testing temperature represents a temperature of -55.degree. C. as specified by the military specification with a 3.degree. C. guard band.
QABO (-55.degree. C.)
Similar to the previous QABO test, this QABO test runs a sample of the units 106 through the test sequence at the non-guardbanded cold temperature. If any parts fail this test, then the entire lot must be retested, as explained above.
MARK and PACK
All parts that pass the previous tests are marked with the appropriate part number and packed in boxes.
FQA (25.degree. C.)
A sample of units are unpacked for testing at room temperature according to the test sequence. If all of the sampled parts 106 pass FQA, then they are repackaged and shipped. By the time a part 106 reaches FQA, it theoretically must have passed all the other tests. If a part fails FQA, the failure may indicate that the testing procedure is faulty, and thus the entire lot must be rescreened through the test flow.
A common cause of disruption in the test flow is misdirection of a part into the wrong bin during Binout. Misdirection may occur due to a mechanical problem in the handler whereby the part falls into the wrong collection bin tube. Further, even though the part may fall into the correct collection bin tube, the tube itself may be manually misplaced into the wrong bin container by the test equipment operator. Thus, when parts are removed from the bin and placed into the loading bin tubes 108 for the next test step of the test flow, a part that has failed the previous test may actually have been erroneously passed on to the next test. This would result in a failure at FQA, requiring expensive rescreening of the entire lot. Alternatively, a part that has not been tested at all may erroneously be passed on to the next test. Finally, a part from one passing category may be treated as belonging to another passing category, and thus miscategorized for the remaining tests in the test flow.
Based on the foregoing, it should be appreciated that it is desirable to guarantee that by the time a part has reached final quality assurance testing, the part has passed through each test of the test flow. By doing so, the need to rescreen the entire lot due to faulty test processing is minimized.